The present invention relates to a process for the plasma-catalytic production of ammonia.
Ammonia synthesis is one of the most important industrial processes in chemistry. Ammonia is produced on an industrial scale from hydrogen and nitrogen by the well-known Haber-Bosch process.
Apart from the use for the preparation of fertilizers, ammonia is also becoming increasingly important in the removal of nitrogen oxides from waste gases containing oxygen by selective catalytic reduction of nitrogen oxides (SCR process; selective catalytic reduction), wherein the nitrogen oxides are reacted with ammonia on a suitable catalyst to nitrogen and water.
For the selective catalytic reduction of nitrogen oxides contained in lean exhaust gases of motor vehicles, simple processes for producing ammonia are required on board the vehicle. Apart from production by hydrolysis of urea, processes employing electrical gas discharges would be advantageous, since chemical reactions may be excited in such plasmas at low temperatures which are otherwise possible only at appreciably higher temperatures.
Kameoka et al. (xe2x80x9cFormation of novel Al2O3 surface (Alxe2x80x94O*) by plasma-excited nitrogen and its catalytic applicationxe2x80x94production of ammonia and oxygen from nitrogen and waterxe2x80x9d; Applied Surface Science 121/122(1997) 351-354) observed the formation of ammonia by reaction of excited nitrogen molecules N2* with the hydroxyl groups on the surface of the alumina. The nitrogen molecules were excited in a high-frequency gas discharge (13.56 MHz, 60 W, 1 torr). The ammonia formed and adsorbed on the surface was desorbed and detected by heating the alumina in helium to 450xc2x0 C. A second treatment of the alumina in the nitrogen plasma led to a drastic reduction in ammonia formation due to hydroxyl group depletion on the surface. It was not possible to restore the hydroxyl groups on the surface of the alumina by treatment in water vapor. The authors attributed this to the formation of a new surface state Alxe2x80x94O* of the alumina due to the excited nitrogen. The excited nitrogen is composed of radicals (N2*,N*) and/or ions (N2+, etc):
N2* +6Alxe2x80x94OHxe2x86x92xe2x86x922NH3+6Alxe2x80x94O*xe2x80x83xe2x80x83(1)
In reaction equation (1), only N2* is given to represent all the possible excited nitrogen components. The original hydroxylated surface state of the alumina could be restored only by treating the alumina in water (liquid). In so doing, cleavage of oxygen occurred, in accordance with the following reaction equation:
6Alxe2x80x94O*+3H2Oxe2x86x92xe2x86x926Alxe2x80x94OH+1.502xe2x80x83xe2x80x83(2)
The following equation for the process as a whole is obtained from reaction equations (1) and (2):
N2*+3H2Oxe2x86x92xe2x86x922NH3+1.502xe2x80x83xe2x80x83(3)
In the summation equation (3) it should be borne in mind that the process described thereby does not take place continuously but requires a constant switching between ammonia formation and hydroxylation on the surface of the alumina. Moreover, this process takes place not at normal pressure but at a pressure of only 1 torr. Moreover, the ammonia formed must be desorbed from the surface at elevated temperatures.
An object of the present invention was, therefore, to enable the production of ammonia continuously and at low temperatures with the aid of an electrical gas discharge plasma.
The above and other objects can be achieved according to the present invention by passing a gas stream containing nitrogen and water vapor through an electrical gas discharge in the discharge space of which is arranged a catalyst which contains a catalytically active component comprising at least one metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, manganese and copper on a support. Mixtures thereof can also be used.
The process according to the invention may also be described by reaction equation (3). Unlike the known process, however, the formation of ammonia in the present process takes place continuously. To this end, nitrogen and water vapor are passed through the gas discharge simultaneously. This alone, however, does not lead to success. Rather, an important factor for the process is the arrangement of a suitable catalyst in the gas discharge.